Thermal stability enhancement of hydrogen bonded semicrystalline thermoplastics achieved by combination of aramide chemistry and supramolecular chemistry

Abstract

Thermo-stability enhancement of supramolecular thermoplastics is a meaningful task to enable more application for their usage at high temperatures. An inspiration from aromatic polyamide (aramide) led us to prepare multi-block semicrystalline supramolecular polymers bearing an amorphous dicarboxylic central block and end caps composed of a hydrogen bonding unit (UDETA) and an aramide unit (NHCO-phenyl ring-OCHN). We also prepared three other semicrystalline materials to demonstrate thermal property tuning by simply varying the end caps. The thermal properties of the semicrystalline compounds, including glass transition temperature (T_g) and melting point (T_m) measured by differential scanning calorimetry (DSC), decomposition temperature (T_d) measured by thermogravimetric analysis (TGA), and softening temperature (T_softening) measured by dynamic mechanical analysis (DMA), were systematically varied depending on the thermal features of the end caps. Especially, T_m and T_d were found to be higher than 200 °C and 400 °C, respectively, when both hydrogen bonding and aramide fragments were present. Melt viscosity investigated by rheology was lower than 1 Pa s for each compound due to the low molecular weight of the components (nearly 1200 g mol⁻¹ or less), which is desirable for practical melt processing. Interestingly, shear thinning behavior was observed only in the aramide unit incorporated compound among all the compounds. This indicates a highly clusterable nature of the aramide fragments, which was also supported by the data of its larger flow activation energy than other compounds bearing no aramide fragments. Between T_g and T_m, a large elastic plateau region was observed in the DMA measurements for all compounds, despite their low molecular weight, meaning that the flexible strands are effectively connected into a network through crystallized end-capping groups.